Season 5 of Buffy is one of those amazing examples when all the story-telling elements come together to create a unified and explosive plotline arc. This season has everything: a deposed God, mideval knights, the watchers council, monks, lots of dark witchcraft, Dracula, two Xanders, the newly arrived little sister that turns out to be timeless destructive energy, Spike fighting for the good guys, the First Slayer, crazy robots, brain-sucking, aliens from outerspace, a really cute doctor, and the deaths of two main characters!!
Glory is by far (in my opinion) the best big bad ever! She's fantastically powerful, but at the same time really ditzy. She fits perfectly into the Buffy-verse: scary & funny, but believable all at the same time. This season also cements in Tara and Spike as scoobies. And it introduces Warren for the first time (who will figure largely into the plot of Season 6).
The highlights of the Season: Fool for Love (Spike's past), The Body (realistic look at death), Tough Love (first glimpse at Dark Willow), and The Gift (my favorite season finale & the death of another main character).
And as with all good things, there are some low points. Listening to Fear (alien is summoned to kill crazy people), and Family (Tara's creepy family arrive) aren't the best, but they're still better than most of what's on TV. Bad Buffy is a lot like bad pizza: even when it's bad, it's still pretty good! OK--corny, I know, but it makes a point! Also, I'm a huge Riley (Marc Blucas) fan, and was sad to see him go this season (he goes on to make a guest appearance in Season 6).
In conclusion, buy Buffy Season 5 on DVD!!! Buy several--buy them as holiday presents!! People will be glad you did!